fix: prevent scroll on landing page (#5595)

use hook

src/pages/Landing/index.tsx

 import { BaseButton } from 'components/Button'
 import { LandingPageVariant, useLandingPageFlag } from 'featureFlags/flags/landingPage'
 import Swap from 'pages/Swap'
+import { useEffect } from 'react'
 import { useLocation } from 'react-router-dom'
 import { Link } from 'react-router-dom'
 import { useIsDarkMode } from 'state/user/hooks'
@@ -130,7 +131,21 @@ export default function Landing() {
   const location = useLocation()
   const isOpen = location.pathname === '/'
 
-  if (useLandingPageFlag() === LandingPageVariant.Control || !isOpen) return null
+  const landingPageFlag = useLandingPageFlag()
+
+  useEffect(() => {
+    if (landingPageFlag) {
+      document.body.style.overflow = 'hidden'
+      return () => {
+        document.body.style.overflow = 'auto'
+      }
+    }
+    return () => {
+      // need to have a return so the hook doesn't throw.
+    }
+  }, [landingPageFlag])
+
+  if (landingPageFlag === LandingPageVariant.Control || !isOpen) return null
 
   return (
     <>